When open-circuit detection is performed in a circuit, in which power supply to a load is repeatedly turns on and off, an open circuit may be erroneously detected in a condition that the power supply to the load is turned off. For instance, erroneous open circuit detection may occur if turn-on and turn-off of a load is controlled via pulse width modulation (PWM) control. No current flow through the load during a period that a PWM signal is low. If open-circuit is determined based on whether current flows through the load and the open-circuit determination is performed during this period, an open circuit is erroneously detected because no current flow through the load.
Furthermore, the open-circuit detection is performed by a CPU in synchronization with pulses of the PWM signal, namely, an interrupt operation is required every time a pulse of the PWM signal appears. As a result, processing in the CPU becomes complicated and processing time increases.
In this circuit, the open-circuit determination is performed based on a voltage across the load. Therefore, this circuit cannot be applied for detecting an open-circuit in a light emitting diode (LED) indicator system having multiple lines of LEDs, which are connected in parallel to each other. If a part of the multiple LED lines becomes open, the circuit cannot detect the open.
To solve the erroneous open-circuit determination problem, an open-circuit detecting circuit that performs open-circuit determination on a rising edge and a falling edge of a PWM signal is proposed in JP-A-9-89966. However, this circuit still produces an erroneous open-circuit detection result if a MOSFET is used for a driving element. A MOSFET produces a delay between time when a PWM signal becomes high and time when its output becomes high. This delay is referred to as a turn-on delay. An open circuit is erroneously determined during a period of the turn-on delay because an output of the MOSFET is still low if the open-circuit determination is performed only when the PWM signal is high.
A PWM control device that does not perform open-circuit determination during a period of the turn-on delay is proposed in JP-B2-3230471 to solve the above-described problem. However, extra processing is required for waiting until the period of the turn-on delay expires, and therefore a load on a CPU increases. To reduce the load on the CPU, a delay circuit is required. Thus, the number of circuits and components of the PWM control device increases. The same kinds of problem occur in shorted load detection.